You can't concentrate.

Chronic physical and mental exhaustion can lead to various cognitive problems, such as an inability to concentrate, hold attention, and remember things.You'll find that it's harder to get things done and you simply can't keep up the way you used to.

You feel guilty.

You're constantly working, but can't seem to get all of your work done (maybe because your work load is too heavy or you can't concentrate) and you eventually feel guilty for not completing your work, which leads to working even more.

Your mood changes often.

A report published by the NASW Assurance Services, Inc., says that burnout may cause emotional exhaustion and a loss of a sense of personal accomplishment, and therefore lead to depersonalization, alienation, and depression.

You keep calling in sick.

In an article on Everyday Health, Edwin R. Shirley, PhD, assistant professor of psychiatry at Case Western Reserve University, says that "when you just don't have the energy or interest to face your workday, it could be a sign of job burnout." This means that these workers are "often late for work, frequently call in sick and procrastinate on the job."

You drink more often than usual.

If you find yourself at happy hour a bit too much, this could be a coping mechanism to avoid all the feelings that are weighing you down. The Mayo Clinic asks potentially burnt out workers if they are using food, drugs, or alcohol.

If not managed carefully, this stress reliever could turn into an addiction.